Machining apparatus and methods

ABSTRACT

A machining spindle comprising an inner shaft ( 110   a ) arranged for carrying a first tool ( 5   a ) for machining a work-piece and an outer shaft ( 110   b ) arranged for carrying a second tool ( 5   b ) for machining the workpiece, wherein the shafts are mounted one inside the other for rotation about a common axis and for axial movement relative to each other. The shafts are supported by air bearings. The spindle is particularly suited to dicing silicon wafers into oblong chips.

This invention relates to machining apparatus and methods andparticularly to apparatus and methods for use in grinding and/or cuttingworkpieces, for example semiconductor wafers.

There are a large number of different circumstances where it isnecessary to machine workpieces. The apparatus and methods described inthe present application are applicable for use in a broad range of suchcircumstances. However, the apparatus and methods of the presentapplication are particularly useful in dicing workpieces such assemiconductor wafers and arrays of chip scale packages for supportingcompleted chips. Some of the introduction and specific description inthis specification will be directed to such dicing apparatus andmethods. It, of course, should be borne in mind that the apparatus andmethods in general and the described construction of the machiningspindle are also appropriate for use in a large number of othermachining operations.

In the production of semiconductor chips it is commonplace to start witha semiconductor wafer which is then appropriately processed to providethe necessary circuits for a plurality of chips arranged in an array onthe wafer. So called “streets” are left between the circuits for eachchip. These streets contain no circuitry and are arranged such thatthere is a first set of parallel streets running in a first directionand a second set of parallel streets running perpendicularly to thefirst set. These streets provide a region which can be cut to allow thewafer to be cut up or “diced” into the individual chips.

The resulting chips are often mounted in chip scale package elements sothat they can be more easily handled and inserted into the appropriatecircuit boards for their intended use. Chip scale package elements arealso often made by dicing a sheet containing an array of packageelements. The methods and apparatus of the present application may alsobe used in dicing such sheets of chip scale package elements. Moreover,in at least some circumstances, one machine may be used for the twodicing operations.

When dicing wafers it is often the case that the chips have an oblongshape rather than a square shape. This, of course, means that thestreets running in one direction are spaced by a different distance thanthe streets running in the other direction. Any machine used to dice thewafer must be able to cope with this difference in spacing.

In the simplest dicing machines a single cutting tool is used and thistravels back and forth across the wafer cutting one street at a time ina first direction. Once all of the streets in a first direction havebeen cut the workpiece is typically rotated through 90° so that thecutting tool can be used to cut all of the streets in the seconddirection.

However, in an effort to improve efficiency, systems have been devisedwhere a plurality of cutting tools are used such that the wafer may becut along a plurality of streets simultaneously.

In existing systems where a plurality of streets can be cutsimultaneously there is typically at least one of the following twoproblems.

In a first set of existing systems a multiple cutting head is providedwhich is driven by one shaft. In such systems the spacing between eachcutting tool cannot be altered without replacing the cutting head as awhole. This renders impractical, or at least significantly slows down,an operation where a wafer needs to be diced into oblong shapes.

In a second set of existing systems two or more separate cutting toolsare provided each of which is separately driven by its own shaft andmotor etc. Such a system can be made to cope with dicing oblong chipsbut there is a disadvantage in that two complete sets of supporting anddriving apparatus are required and there can be difficulties inmaintaining accuracy between the two cutting tools.

It is an object of the present invention to provide machining apparatusand methods which alleviate at least some of the problems associatedwith the prior art.

According to one aspect of the present invention there is provided amachining spindle comprising a first shaft arranged for carrying a firsttool for machining a workpiece and a second shaft arranged for carryinga second tool for machining the workpiece, wherein the shafts aremounted for rotation about a common axis and for axial movement relativeto each other.

This arrangement allows the tools to be operated accurately relative toone another whilst avoiding the need for the use of two separatespindles and their associated equipment.

Preferably the shafts run one inside the other so that the first shaftis an inner shaft and the second shaft is an outer shaft.

The spindle may comprise a main body within which the shafts arejournalled. In currently preferred embodiments the inner shaft ismounted within the outer shaft which in turn is journalled within themain body. The inner shaft may be journalled within the outer shaft toallow relative rotation between the two shafts.

Preferably a bearing is provided to allow relative movement between theinner and outer shafts. Typically this bearing is arranged to allowrelative axial movement. This bearing may also be arranged to allowrelative rotation between the inner and outer shafts. The arrangement ofthe spindle as a whole is typically such that one of the shafts may bearranged to move axially relative to the main body.

Air bearings may be provided to support the shafts. The main body maycomprise jets to provide air to a bearing allowing relative rotationbetween the main body and the outer shaft. The inner shaft may comprisejets to provide air to the bearing allowing relative movement betweenthe inner and outer shafts.

The air bearings may be arranged such that air is purged from thespindle at positive pressure (relative to the ambient pressure) at alllocations which may be exposed to the by-products of machiningoperations. This can help to ensure that dust, swarf etc does not enterthe bearings.

In an alternative to this or in addition to this, supplementary sealingmeans may be provided. Such a seal may be a labyrinthine seal.

“Air bearings” per se, where a gas provides support and lubrication, arewell known. Whilst the gas will normally be air, as is conventional, theexpression “air bearing” in this specification also covers bearingsusing or designed to use other gases in place of, or in addition to air.

The spindle may comprise at least one electric motor for rotatinglydriving the shafts.

In one set of embodiments the spindle is arranged to allow the firstshaft to rotate at a different speed from and/or in a opposite directionfrom the second shaft. The spindle may comprise two electric motors, arespective one of the motors for rotatingly driving each shaft.

In another set of embodiments the spindle is arranged so that the firstand second shafts rotate in synchrony with one another. In suchembodiments drive transfer means may be provided for transferring drivefrom one shaft to the other. The drive transfer means may comprise a pinmounted on one of the shafts and disposed in a recess in the other ofthe shafts such that shafts may move axially relative to one anotherwithout interrupting the transfer of drive.

The spindle may comprise axial drive means for axially driving theshafts relative to one another. The axial drive means may be arranged toact on one of the shafts via an axial bearing assembly. The axialbearing assembly may be arranged for axial movement within an airbearing provided in the main body. A portion at an end of one of theshafts may be captured in the axial bearing assembly.

Encoding scale means may be provided to indicate the axial position ofthe shaft which is moveable axially relative to the main body. Theencoding scale means may, for example, be provided on the axial bearingassembly or the axial drive means.

The machining spindle may be a cutting and/or grinding spindle arrangedfor supporting cutting and/or grinding tools.

The machining spindle may be a dicing spindle, the shafts each beingarranged for supporting a respective cutting wheel. The workpiece insuch a case may, for example, be a semiconductor wafer or a sheetcomprising an array of chip scale package elements.

Where two cutting wheels are used they may be different from one anotherin diameter and/or other properties. The cutting wheels may be used in atwo stage cutting process. One of the cutting wheels may be a V-cutterfor use in making a first cut.

The grinding tools may be cup grinders for grinding a surface by axiallymoving the tool into contact with the workpiece. The grinding tools maybe radial grinders. The radial grinders may be arranged for use in formgrinding a complex shape. The grinding tools may be arranged so that thefirst tool can grind the internal surface of a bore whereas the secondtool can grind the external surface of the component having the bore.

According to another aspect of the invention there is provided amachining apparatus comprising a machining spindle as defined in any ofthe aspects above and a support arrangement for supporting the spindle.

The machining apparatus may further comprise a workpiece table arrangedfor supporting a workpiece during machining.

The apparatus may further comprise a first tool mounted on the firstshaft and a second tool mounted on the second shaft. The first andsecond tools may, for example, be cutting wheels, grinding tools etc asexplained above.

According to a further aspect of the invention there is provided amethod of machining a workpiece comprising the step of using a machiningspindle or a machining apparatus as defined in any one of the aspectsabove.

In one particular application of a machining spindle or apparatus, theability to move one shaft axially relative to the other may be used tocompensate for thermal growth, or more particularly differences ofthermal growth, in the shaft or other components as they heat up due tooperation. In such a method the spacing between the carried tools, forexample cutting wheels, may be monitored and the shafts moved relativeto one another in an attempt to keep the spacing constant.

The present invention will now be described by way of example only withreference to the accompanying drawings in which:—

FIG. 1 is a schematic end view of a machining apparatus;

FIG. 2 is a side view of the machining apparatus of FIG. 1;

FIG. 3 is a sectional view on line III-III of the machining spindle ofthe apparatus shown in FIG. 1;

FIG. 4 shows an alternative machining spindle which may be used in themachining apparatus shown in FIG. 1;

FIG. 5 shows an alternative machining spindle which may be used in themachining apparatus shown in FIG. 1; and

FIG. 6 shows part of the alternative machining shown in FIG. 5.

FIGS. 1 and 2 show a machining apparatus comprising a machining spindle1 supported by a spindle support carriage 2 which is arranged forsupporting the machining spindle 1 and allowing translational movementof the spindle in three perpendicular directions one of which isparallel to the axis of the spindle 1. The apparatus also includes aworkpiece table 3 upon which a workpiece 4 may be supported formachining. The workpiece table 3 is arranged so as to be rotatable aboutan axis perpendicular to the surface on which the workpiece issupported, i.e. an axis in the plane of the page as shown in FIGS. 1 and2. The machining spindle 1 carries a pair of cutting wheels 5 a, 5 bwhich are spaced from one another in a direction parallel to the axis ofthe spindle 1.

In operation, these cutting wheels 5 a, 5 b may be brought into contactwith the workpiece 4 by moving the machining spindle 1 on its carriage 2in the appropriate direction (the vertical direction in FIGS. 1 and 2).The cutting wheels 5, 5 b may then be drawn across the workpiece 4 inorder to create cut or score lines across the workpiece 4 in a firstdirection. This process may be repeated as many times as is desiredacross the workpiece 4 so as to cut the workpiece into strips. Theworkpiece table 3 may then be rotated about the axis defined above suchthat its orientation is changed by 90°. This rotation will of courseoccur with the cutting wheels 5 a, 5 b out of contact with the workpiece4. Once the workpiece table 3 and workpiece 4 have been rotated theworkpiece 4 can be cut in a perpendicular direction to the first cuts,again using the cutting wheels 5 a, 5 b. It will be appreciated thatthis process serves to dice the workpiece 4.

In one particular application, the workpiece 4 may be a semiconductorwafer and the cutting wheels 5 a, 5 b may be used to cut along thestreets in the wafer so as to dice the wafer into the appropriate chips.

As will be explained in more detail below, the spacing between thespaced cutting wheels 5 a, 5 b may be changed by virtue of theconstruction of the machining spindle 1. The ability to change thespacing between the cutting wheels 5 a, 5 b may be used in a number ofdifferent ways. Perhaps most typically this ability to change thespacing between the cutting wheels 5 a, 5 b can be used to dice aworkpiece into oblong elements, for example, oblong chips. In such acase the cutting wheels 5 a, 5 b will be used to cut the workpiece in afirst direction whilst they are spaced with a first spacing and then thespacing between the cutting wheels 5 a, 5 b will be changed beforecutting in the second direction so that the second set of cuts have adifferent spacing.

It should be noted that there is no need for the cutting wheels 5 a, 5 bto cut adjacent streets or other cut lines in a single traverse of theworkpiece 4. Especially in the case of semiconductor dicing it may bemore common that, say, the first and third or the first and fourthstreets are cut with a first pass and then the second and fourth orsecond and fifth are cut with a second pass and so on. The reason forchoosing such a cutting technique is simply that there will be aphysical limit on how small the spacing can be made between the cuttingwheels 5 a, 5 b.

FIG. 3 shows the machining spindle 1 of the apparatus shown in FIGS. 1and 2. Firstly it should be noted that as shown in FIG. 1, the main body100 of the machining spindle has a non-circular shape. In particular,there are cutaways from the circular shape at the region of the spindle1 where it comes closest to the workpiece table 3. This configuration isto allow the spindle 1 to pass over the top of a supported workpiece 4without unnecessarily increasing the diameter of the cutting wheels 5 a,5 b or otherwise compromising the performance of the apparatus. It willbe noted that the section of the spindle 1 shown in FIG. 3 is takenalong a line where the spindle 1 has its full diameter. The minimumradial dimension of the spindle 1, due to the cutouts, is indicated bythe dotted line C shown in FIG. 3.

The main body 100 of the spindle 1 houses an inner shaft 110 a, at thedistal end of which is provided a hub 111 a for carrying the firstcutting wheel 5 a (not shown in FIG. 3). This inner shaft 110 a isjournalled for rotation about the central axis of the machining spindle1 inside an outer shaft 110 b. The outer shaft 110 b carries a hub 111 bat its distal end for supporting the second cutting wheel 5 b (not shownin FIG. 3). The outer shaft 110 b is journalled for rotation about thecentral axis of the machining spindle 1 inside the main body 100. Thus,the outer shaft 110 b is generally a hollow cylinder within which theinner shaft 110 a is disposed.

The inner shaft 110 a has an extension portion 112 a which extendsthrough the proximal end of the outer shaft 110 b. The rotor 120 of anelectric motor for driving the shafts 110 a, 110 b is mounted on acollar 112 b of the outer shaft 110 b which surrounds this extensionportion 112 a. The stator of the electric motor 121 is mounted in themain body 100.

The extension portion 112 a of the inner shaft 110 a terminates in adisc like portion 113 a which is captured in a moving axial bearingassembly 130.

Whilst the disc like portion 113 a is captured in the moving axialbearing assembly 130, air bearings exist around the disc like portion113 a such that rotation of the disc portion 113 a and hence the innershaft 110 a as a whole is not prevented or substantially hindered.

On the other hand, the moving axial bearing assembly 130 is arranged foraxial movement within the main body 100 and as this axial movementoccurs, a corresponding axial movement of the inner shaft 110 a iscaused to occur due to the disc like portion 113 a being captured in themoving axial bearing assembly 130.

An axial driving means 131 is provided for driving the axial bearingassembly 130 in the axial direction. The axial driving means 131 isarranged to be able to drive the axial bearing assembly, and hence theinner shaft 110 a in both directions along the axis of the spindle 1.

In contrast, the outer shaft 110 b is held against axial movementrelative to the main body 100 by an axial bearing plate 101 provided atthe distal end of the main body 100. Thus, by operating the axialdriving means 131 to move the axial bearing assembly 130 and hence theinner shaft 110 a, the spacing between the hubs 111 a and 111 b andhence the cutting wheels 5 a and 5 b can be altered.

The above description covers the main features which are directed at theprinciple of operation of the machining spindle 1. FIG. 3 shows moredetail of one practical implementation of such a system and some ofthese details as well as others will be described below.

The electric motor comprising the rotor 120 and stator 121 can be usedto rotatingly drive the outer shaft 110 b and furthermore control means(not shown) can be used to sense and control the speed of rotation. Thisis particularly important when the cutting wheels 5 a, 5 b are broughtinto contact with the workpiece 4.

Although not shown, drive transfer means is provided between the outershaft 110 b and the inner shaft 110 a so that the motor 120, 121 canalso drive the inner shaft 110 a. This drive transfer arrangement needsto transfer the drive as the relative axial positions of the first andsecond shafts 110 a and 110 b are changed. One appropriate drivetransfer means comprises a pin mounted on one of the shafts 110 a, 110 band disposed in a recess in the other shaft 110 a, 110 b such that therotating drive is transferred but axial movement is not opposed. In apreferred form the pin may be parallel to, but spaced from, the axis ofrotation and arranged to run in an appropriate bore in the respectiveother shaft. If it is desired, a plurality of such pins might beprovided. In another alternative, radial pins or splines might be used.

The outer shaft 10 b is supported for rotation in a spaced pair of airbearings 102 a and 102 b provided in the main body 100. The main body100 comprises internal drillings for supplying air to these air bearings102 a, 102 b and exhausting spent air from the air bearings.

A seal bearing 103 is provided between the two supporting bearings 102a, 102 b. This sealing bearing 103 is provided to seal, as effectivelyas is possible, an air passage 104 which runs from the main body 100through the outer shaft 110 b and into the centre of the inner shaft 110a. A seal bearing 103 a is also provided on the inner shaft 110 a aboutthe point where the air passage 104 meets the inner shaft 110 a. It willbe appreciated that whilst an air passage 104 has been described, inactual fact there will be a plurality of drillings through both theouter shaft 110 b and the inner shaft 110 a to provide suitable pathsfor air as the two shafts are rotating.

The inner shaft 110 a is a jetted shaft and internal drillings 114 aleading to jets 115 a to the external surface of this shaft 110 a areprovided to feed air to the gaps between the shafts 110 a, 110 b suchthat the inner shaft 110 a runs on an air bearing within the outer shaft110 b.

The moving axial bearing assembly 130 is supported for axial movement inan air bearing 105 provided in the main body 100.

The axial bearing assembly 130 comprises inner drillings (not shown) forfeeding air from the supporting air bearing 105 to the air bearingswhich support the disc like portion 113 a of the inner shaft 110 a.

Air is purged from the spindle under positive pressure at the region Pwhere the inner and outer shafts 110 a and 110 b penetrate through themain body 100. This helps to ensure that external contaminants such asswarf or the by-products of sawing do not enter the spindle 1 where theywould risk fouling the air bearings.

Carbon contact brushes 106 a and 106 b are provided in the axial bearingassembly 130 for contacting with the disc like portion 113 a at the endof the inner shaft 110 a. There is a complete electrical conduction pathbetween the cutting wheels 5 a, 5 b and these brushes 106 a, 106 b viathe metal of the shafts 110 a and 110 b. Thus, if leads from the carbonbrushes 106 a and 106 b are connected to an appropriate detector (notshown), then provided that the cutting wheels 5 a and 5 b are conductiveand an appropriate contact is made with a (conductive) workpiece 4 acircuit will be made as the cutting wheel 5 a, 5 b comes into contactwith the workpiece. The detector can be used to sense the making of thiscircuit to determine that the blades 5 a, 5 b have touched down incontact with the workpiece 4.

It is envisaged that the machining spindle will be operated at cuttingspeeds in the order of 40,000 to 60,000 rpm and that the axial movementdesired and provided by the axial bearing assembly 130 will be in theorder of 6 to 7 mm. However, these figures only represent what might betrue in respect of a typical machine and the present invention is in noway restricted to such values.

The axial drive means 131 or the axial bering assembly 130 is providedwith an encoding scale to show the axial position of that component andhence the inner shaft 110 a so that the spacing between the cuttingwheels 5 a, 5 b can be accurately determined. In one particularapplication of the present spindle the facility for moving the shafts110 a, 10 b relative to one another in an axial direction may be used tocompensate for differential thermal growth in components of the spindle1 during operation.

Typically, the shafts 110 a, 110 b will increase in length due toheating caused by high speed rotation. Whilst these changes of lengthand the differences in changes of length may be very small, they maystill be important. The tolerances which must be met to properly dicesemiconductor wafers, for example are very tight and this facility forthermal compensation can be particularly useful.

In a further application the cutting wheels 5 a, 5 b may be differentfrom one another. For example, one of the wheels may be a V-cutter forscoring or making a first cut in the workpiece 4, whereas the secondwheel may be chosen so as to be suitable for completing the cuttingoperation.

In an alternative rather than relying solely on positive air pressurepurging to protect the spindle 1 against the ingress of foreignparticles, a labyrinthine seal may be provided to give furtherprotection.

FIG. 4 shows an alternative spindle 1′ which in many respects is similarto that shown in FIG. 3 and described above. Therefore, for the sake ofbrevity the same reference numerals are used for corresponding portionsand a detailed description of these is omitted.

The main difference between the spindle shown in FIG. 4 and that shownin FIG. 3 is that an additional motor comprising a rotor 120 a mountedon the inner shaft 110 a and a corresponding stator 121 a mounted in themain body 100 is provided in addition to a motor of the same typedescribed above comprising a rotor 120 provided on the outer shaft 110 band a respective stator 121 mounted in the main body 100.

Thus, in the spindle 1′ shown in FIG. 4 there are two electric motors,one of which 120 a, 121 a is used to drive the inner shaft 10 b and theother of which 120, 121 is used to drive the outer shaft 110 b.Therefore, no drive transfer means is required between the inner andouter shafts 110 a and 110 b.

Moreover, the speed and direction of rotation of the shafts 110 a, 110 band hence the cutting wheels 5 a and 5 b can be controlled independentlyfrom one another. Thus, in some instances, one of the cutting wheels 5a, 5 b can be run at a different speed from the other or if it isdesired, a first of the cutting wheels may be run in one direction and asecond in the opposite direction.

One particular scenario where being able to run the blades in oppositedirections may be of use, is where it is desired to perform cuts in bothdirections as the spindle traverses the workpiece. Similarly, being ableto run the shafts at different speeds may be of assistance where the twocutting wheels 5 a, 5 b are different from one another and requiredifferent speeds of rotation for optimum performance.

Having said this, the spindle of FIG. 4 presents more productiondifficulties than that of FIG. 3 and hence such a spindle would be moreexpensive to produce and may suffer from performance degradation in somerespects. As can be seen in FIG. 4, the inner shaft 110 a in thisspindle has greater length than that in the spindle shown in FIG. 3 andprojects a significant distance beyond the region in which it issupported by the outer shaft 10 b. Furthermore, significant mass in theform of the rotor 120 a is mounted on the shaft at this extendedportion. These factors will tend to make it harder to achieve smoothrunning with the spindle 1′ shown in FIG. 4 and may mean that thecutting rotational velocities, at least of the inner shaft 110 a, mustbe reduced. Thus, for example, a rotational speed in the range of 28,000to 40,000 rpm may be more manageable with the spindle 1′ of FIG. 4.

It will be noted that the shape and dimensions of the motors 120, 121,121 a, 120 a provided for the outer and inner shafts 110 b and 110 a aredifferent from one another. In colloquial terms one can be termed asbeing short and fat whilst the other one is long and thin. These shapeshave been chosen in an effort to ensure that the motors can deliver thesame or a similar power for use in rotation whilst best occupying thespace available and minimising any adverse effects on the spindle'sperformance.

FIG. 5 shows a further alternative spindle 1″ which is a development ofthe spindle shown in FIG. 3. The spindle shown in FIG. 5 is similar inmost currently important respects to that shown in FIG. 3 and therefore,for the sake of brevity, detailed description of these elements isomitted and the same reference numerals are used to indicate the sameparts as in FIGS. 3 and 4.

The spindle shown in FIG. 5 includes drive transfer means as does thespindle shown in FIG. 3, but in FIG. 4 the drive transfer means areshown. The drive transfer means comprise a diametrically opposed pair ofdrive pins 1001. Each drive pin has a stem portion 1001 a which islocated in a closely fitting recess 1002 provided in the inner shaft 110a and a head portion 1001 b located in a slot like aperture 1003 in theouter shaft 110 b. The slot like aperture 1003 is dimensioned so as toclosely fit the head portion 1001 b of the respective drive pin 1001 inthe circumferential direction but to be considerably larger than thehead portion 1001 b in the axial direction. This means that drive can betransferred from the outer shaft to the inner shaft via the pins 1001acting as a drive transfer means but relative axial movement between thetwo shafts 110 a and 110 b is not impeded, because during this axialmovement, the head portions 1001 a can slide within the slot likeapertures 1003.

The drive pins in this embodiment are provided so as to be insulating,that is to say, so that there is no electrical conduction path from theinner shaft 110 a to the outer shaft 110 b via the drive pins 1001. Insome cases this may be achieved by using insulating drive pins but inthe present embodiment this is achieved by the head portions 1001 a ofthe drive pins 1001 having ceramic (i.e. non-insulating) covers. Theprovision of insulating drive means is useful because it means that thespindle as a whole may be constructed so that during operation there isno electrical conduction path between the inner and outer shafts 110 a,110 b. This in turn facilitates the detection of tool touchdown on toconducting or semi-conducting work pieces.

Furthermore, to help in the electrical detection of tool touchdown, inthe present embodiment, insulating sleeves 1004 are provided on theinternal surface of the collar 112 b of the outer shaft which surroundsthe extension portion 112 a of the inner shaft. These insulating sleeves1004 serve as guide bearings for supporting the extension portion 112 aof the inner shaft 110 a and at the same time help to maintainelectrical isolation between the inner shaft 110 a and the outer shaft110 b.

Further, in the present embodiment, the brushes used to contact with theshafts 110 a and 110 b in the touchdown detection method are differentfrom those in the embodiment shown in FIG. 3. In the embodiment of FIG.5, one brush contacts with the disc like portion 113 a at the end of theinner shaft 110 a and a further brush is arranged for controllablecontact with a shoulder portion 1005 of the outer shaft 10 b at a regionwhere the collar 112 b meets the remainder of the outer shaft 110 b. Thelocation of this second brush B is indicated in FIG. 6 which shows therelevant portion of the spindle shown in FIG. 5. The second brush B ismounted on a spring loaded carrier C which biasses the brush B away fromthe outer shaft 110 b. The carrier C has an associated pressurised airport AP via which pressurised air is supplied to force the brush Bagainst the shaft 110 b when sensing is desired. As soon as sensing iscomplete the air supply is cut and the brush B retracts under action ofthe spring. This arrangement significantly reduces brush wear which is aparticular problem for the brush B which is contacting with a shaftsurface that has a very high tangential velocity. Typically the brush Bwill only be forced against the outer shaft 110 b until touchdown isdetected, after this contact is not required until another touchdownevent (or possibly lift off) is to be monitored Electrical connectionsto the brush 106 a for connection to the inner shaft 110 a via the disclike portion 113 a is provided by way of a brass screw S at the end ofthe axial driving means assembly and is shown in FIG. 5. A wire (notshown) leads along the port AP for connection to the brush B contactingwith the outer shaft 110 b. These electrical connections are fed into adetector which looks for a circuit to be made between the two shafts byvirtue of the tools (5 a, 5 b as shown in FIGS. 1 and 2), carried by therespective shafts 110 a, 110 b, coming into contact with a conducting orsemi-conducting workpiece. This circuit, it will be seen, includes inseries, a first of the shafts, then the workpiece and then the other ofthe shafts. Breaking of this circuit as the tools lift off the workpiececould be similarly detected if desired.

Whilst the above description has been written in terms of the cutting ordicing of workpieces, and in particular semiconductor workpieces, itshould again be noted that the spindles of the present application andinvention are not restricted to such uses. Thus, for example a spindlewhich is similar to that shown in FIG. 3, FIG. 4 or FIG. 5 may be usedin other types of machining operations, for example, other cuttingoperations or grinding operations. By way of example, in the case ofgrinding, the shafts 110 a and 110 b may be used to support axialgrinders or radial grinders. Thus, for example, the shafts 110 a, 110 bmay be used to support cup grinders for axial grinding of a surface. Bythe same token radial grinders of perhaps different diameter may be usedin form grinding of complex shapes. As a further example, one of theshafts may be used to carry a grinder for use in grinding the internalsurface of a bore whereas the other may be used to carry a grinder forgrinding an external surface of the component including the bore.

1-23. (canceled)
 24. A machining spindle comprising an inner shaft arranged for carrying a first tool for machining a workpiece and an outer shaft arranged for carrying a second tool for machining the workpiece, the shafts being mounted for rotation about a common axis and for axial movement relative to each other, and the machining spindle further comprising a main body within which the shafts are journalled, the inner shaft being mounted within the outer shaft which in turn is journalled by means of an air bearing within the main body and there being an air bearing provided to allow relative axial movement between the inner and outer shafts, wherein the spindle comprises a sensor for sensing when the tools carried by the two shafts contact with a conducting or semi-conducting workpiece, the sensor being arranged to sense a current flowing around a path including the workpiece and the two shafts, and the spindle further comprises a drive transfer member for transferring drive from one shaft to the other so that the first and second shafts rotate in synchrony with one another, the drive transfer member being insulated so that it does not offer an electrical conduction path between the two shafts.
 25. A machining spindle according to claim 24 in which the main body comprises jets to provide air to the air bearing allowing relative rotation between the main body and the outer shaft.
 26. A machining spindle according to claim 24 in which the inner shaft comprises jets to provide air to the air bearing allowing relative axial movement between the inner and outer shafts.
 27. A machining spindle according to claim 24 in which the air bearings are arranged such that air is purged from the spindle at positive pressure, relative to the ambient pressure, at all locations which may be exposed to the by-products of machining operations.
 28. A machining spindle according to claim 24 in which at least one supplementary seal is provided.
 29. A machining spindle according to claim 24 in which the spindle comprises at least one electric motor for rotatingly driving the shafts.
 30. A machining spindle according to claim 24 in which the drive transfer member comprises a pin mounted on one of the shafts and disposed in a recess or an aperture in the other of the shafts such that shafts may move axially relative to one another without interrupting the transfer of drive.
 31. A machining spindle according to claim 30 in which the pin is radially mounted.
 32. A machining spindle according to claim 31 in which the pin is formed with at least one of an insulating material and an insulating material coating.
 33. A machining spindle according to claim 24 in which the spindle comprises an axial drive arrangement for axially driving the shafts relative to one another.
 34. A machining spindle according to claim 24 in which an encoding scale is provided to indicate the axial position of at least one of the shafts, which shaft is movable axially relative to the main body.
 35. A machining spindle according to claim 24 in which the sensor comprises at least one brush contacting with one of the two shafts.
 36. A machining spindle according to claim 24 in which the inner shaft is supported by insulating guide bearings.
 37. A machining spindle according to claim 24, said spindle being a dicing spindle for use in dicing semi-conductor wafers, and the shafts each being arranged for supporting a respective cutting wheel.
 38. A machining spindle according to claim 24, said spindle being a grinding spindle arranged for supporting grinding tools.
 39. A machining apparatus comprising a machining spindle according to claim 24 and a support arrangement for supporting the spindle.
 40. A machining apparatus according to claim 39 and further comprising a workpiece table arranged for supporting a workpiece during machining.
 41. A method of machining a workpiece comprising the step of using a machining spindle as claimed in claim
 24. 42. A method according to claim 41 comprising the step of using the ability to move one shaft axially relative to the other to compensate for one of thermal growth and differences of thermal growth, in at least one component of the spindle as it heats up due to operation.
 43. A method of dicing semi-conductor wafers using a machining apparatus comprising a workpiece table for supporting a wafer and a machining spindle comprising an inner shaft carrying a first cutting wheel for machining the wafer, and an outer shaft carrying a second cutting wheel for machining the wafer, wherein the shafts are mounted for rotation about a common axis and for axial movement relative to each other, and the machining spindle comprises a main body within which the shafts are journalled, the inner shaft being mounted within the outer shaft which in turn is journalled by means of an air bearing within the main body, the method comprising the steps of: cutting along streets in one direction on the wafer, having a first street spacing, using the two cutting wheels set at a first wheel spacing; moving the shafts supporting the two cutting wheels axially relative to one another to set the cutting wheels at a second wheel spacing; and cutting along streets in another direction on the wafer, having a second street spacing, using the two cutting wheels set at the second wheel spacing.
 44. A machining spindle comprising an inner shaft arranged for carrying a first tool for machining a workpiece and an outer shaft arranged for carrying a second tool for machining the workpiece, the shafts being mounted for rotation about a common axis and for axial movement relative to each other, and the machining spindle further comprising a main body within which the shafts are journalled, the inner shaft being mounted within the outer shaft which in turn is journalled by means of an air bearing within the main body and there being an air bearing provided to allow relative axial movement between the inner and outer shafts, wherein the spindle comprises sensor means for sensing when the tools carried by the two shafts contact with a conducting or semi-conducting workpiece, the sensor means being arranged to sense a current flowing around a path including the workpiece and the two shafts, and the spindle further comprises drive transfer means for transferring drive from one shaft to the other, the drive transfer means being insulated so that it does not offer an electrical conduction path between the two shafts.
 45. A machining spindle comprising an inner shaft arranged for carrying a first tool for machining a workpiece and an outer shaft arranged for carrying a second tool for machining the workpiece, the shafts being mounted for rotation about a common axis and for axial movement relative to each other, and the machining spindle further comprising a main body within which the shafts are journalled, the inner shaft being mounted within the outer shaft which in turn is journalled by means of an air bearing within the main body and there being an air bearing provided to allow relative axial movement between the inner and outer shafts, wherein the spindle comprises sensor means for sensing when the tools carried by the two shafts contact with a conducting or semi-conducting workpiece, the sensor means being arranged to sense a current flowing around a path including the workpiece and the two shafts, and the spindle further comprises drive transfer means for transferring drive from one shaft to the other so that the first and second shafts rotate in synchrony with one another, the drive transfer means being insulated so that it does not offer an electrical conduction path between the two shafts. 